A confidential record of a meeting between President Bush and Tony Blair before the invasion of Iraq, outlining their intention to go to war without a second United Nations resolution, will be an explosive issue for the official inquiry into the UK’s role in toppling Saddam Hussein.

The memo, written on 31 January 2003, almost two months before the invasion and seen by the Observer, confirms that as the two men became increasingly aware UN inspectors would fail to find weapons of mass destruction (WMD) they had to contemplate alternative scenarios that might trigger a second resolution legitimising military action.

Bush told Blair the US had drawn up a provocative plan “to fly U2 reconnaissance aircraft painted in UN colours over Iraq with fighter cover”. Bush said that if Saddam fired at the planes this would put the Iraqi leader in breach of UN resolutions.

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The president expressed hopes that an Iraqi defector would be “brought out” to give a public presentation on Saddam’s WMD or that someone might assassinate the Iraqi leader. However, Bush confirmed even without a second resolution, the US was prepared for military action. The memo said Blair told Bush he was “solidly with the president”.

The five-page document, written by Blair’s foreign policy adviser, Sir David Manning, and copied to Sir Jeremy Greenstock, the UK ambassador to the UN, Jonathan Powell, Blair’s chief of staff, the chief of the defence staff, Admiral Lord Boyce, and the UK’s ambassador to Washington, Sir Christopher Meyer, outlines how Bush told Blair he had decided on a start date for the war.